The tendency of optical fibers to leak optical energy when bent has been known since the infancy of the technology. It is well known that light follows a straight path but can be guided to some extent by providing a path, even a curved path, of high refractive index material surrounded by material of lower refractive index. However, in practice that principle is limited, and optical fibers often have bends with a curvature that exceeds the ability of the light guide to contain the light.
Controlling transmission characteristics when bent is an issue in nearly every practical optical fiber design. The initial approach, and still a common approach, is to prevent or minimize physical bends in the optical fiber. While this can be largely achieved in long hauls by designing a robust cable, or in shorter hauls by installing the optical fibers in microducts, in all cases the optical fiber must be terminated at each end. Thus even under the most favorable conditions, bending, often severe bending, is encountered at the optical fiber terminals.
Controlling bend loss can also be addressed by the physical design of the optical fiber itself. Thus ring features or trench features, or combinations thereof, are commonly found at the outside of the optical fiber refractive index profiles to control bend losses. See for example, U.S. Pat. Nos. 4,691,990 and 4,852,968, and U.S. patent application Ser. No. 12/583,212, filed Aug. 17, 2009, all incorporated herein by reference.
Bend loss occurs in both single mode and multimode optical fibers. Multimode optical fibers typically are used in communications over shorter distances such as in data centers, enterprise LAN, SAN, etc. The advantage of multimode fiber lies mainly in the ability to couple this fiber with simple and more cost effective sources. In the past these sources were mainly LEDs with the wavelength around 850 nm. Lately, low cost Vertical Cavity Surface Emitting Laser (VCSEL) with vertical resonators have appeared in the market that enable effective coupling between the laser diode and optical fibers. These laser diodes also achieve high modulation rates, e.g., up to 10.3125 Gbps. For 40G/100G in high performance computing, data center and SAN applications, IEEE p802.3ba proposes standards for parallel VCSEL array with individual channel of 10.3125 Gbps, and/or higher speeds up to 25 Gbps, and/or WDM.
Performance issues for optical fibers under bend conditions have generally been considered to involve generalized optical power loss, due to leakage of light from the optical fiber at the location of the bend. In single mode optical fibers general power loss is the primary consideration, because all leakage involves light in the fundamental mode of the optical fiber. However, in multimode optical fiber the modal structure affects the loss, with higher order modes suffering more loss than lower order modes. The combination of higher order and lower order modes in a multimode optical fiber determines the bandwidth, and thus the signal carrying capacity of the optical fiber.
For high bandwidth, the group velocities of the various modes in multimode fibers should be as close to equal as possible. The differential group velocities can be controlled by grading the refractive index of the material comprising the core, which means specifying a functional form of the index as a function of the fiber radius. In a conventional multi-mode fiber, the design goal has been to achieve a α-shape, which is defined as:
                                                                        n                ⁡                                  (                  r                  )                                            -                              n                clad                                                    n              clad                                =                      Δ            ⁡                          (                              1                -                                                      (                                          r                      /                                              r                        core                                                              )                                    a                                            )                                      ,                            (        1        )            where r is the radius of the fiber, rcore is the radius of the core, nclad is the refractive index of the cladding, and α and Δ are free parameters. This is the so-called α-shape profile.
An inherent limitation of the α-shape profile design is that high order modes are not properly compensated due to coupling to cladding modes at the edge of the core. Thus the modal delay of high order modes deviates from low order and medium order modes. For conventional α-shape MMF, such as OM3 and OM4, the differential mode attenuation of high order modes is high, which minimizes impact of high order modes on differential mode delay and eventually bandwidth. OM3 and OM4 are well known MMF performance standards of the Telecommunications Industry Association (TIA). In MMF designed for low bend loss, the same high order modes will have much less differential mode attenuation. Consequently, the impact of differential mode delay on bandwidth cannot be neglected. Thus, method of equalizing modal delay of high order modes are needed for bending insensitive MMF (BIMMF) used in high speed digital transmission. In the current state of the art, high speed transmission for optical data systems is generally considered to be 10 Gbps or greater.
To support high packaging densities anticipated in supercomputing applications, new design concepts for optimizing band-width, relaxing tolerances for VCSEL coupling, and reducing bend loss are needed.